1. Field of the Invention
The present invention relates to noise detection in communications systems. More specifically, the present invention relates to a system and method for detecting burst noise during quadrature amplitude modulation communications.
2. Related Art
Quadrature Amplitude Modulation (QAM) is a known modulation technique for transmitting digital information. In QAM, both the phases and the amplitudes of two sinusoidal signals are modulated to define points or “symbols” in a constellation, each of which conveys digital information. Various “orders” of QAM can be utilized to transmit digital information at different bit rates, such as 16-QAM, 64-QAM, 256-QAM, etc.
FIG. 1 shows an example of a 16-QAM constellation. The 16-QAM constellation has 16 possible points that can be distinguished where each point represents a 4-bit value. The constellation is represented by an X-Y axis, where the X-axis is referred to as the in-phase (I) axis 100, and the Y-axis is referred to as the quadrature (Q) phase axis 102. During QAM modulation, a sine wave and a cosine wave are transmitted together. By definition, the phases of these two signals are shifted by 90 degrees. The magnitude of the in-phase component is represented by the magnitude of the cosine wave at its peaks, which occurs at 0 and 180 degrees. The magnitude of the quadrature phase component is represented by the magnitude of the sine wave at its peaks, which occurs at 90 and 270 degrees. At the peaks of the cosine wave, the sine wave is zero, and vice-versa. The sine and cosine waves are therefore said to be orthogonal since they do not interfere with each other. Therefore, these two waves may be mixed together. By taking two measurements 90 degrees apart, the magnitude of the in-phase and quadrature phase components may be obtained. For example, constellation point 104 in FIG. 1 has an in-phase magnitude (I)=12, and a quadrature phase magnitude (Q)=4. These two values uniquely define a single constellation point.
QAM is currently used in a number of different communications devices, including cable modems. Cable modems (CMs) can be found in both homes and businesses, and are used to transmit and receive digital information (e.g., to access the Internet, view television and/or on-demand video, etc.). Numerous CMs can communicate with a device known as a Cable Modem Termination System (CMTS), which is installed at a central location and used to transmit information to CMs, as well as receive information from CMs. The signal between these devices traverses a communications network that includes both coaxial cable and fiber optic cable, and is known as a Hybrid Fiber-Coax (HFC) network or cable “plant.” The protocol used to communicate between the CMTS and CMs has been standardized by the CableLabs organization and is collectively known as DOCSIS (Data Over Cable Service Interface Specifications). The set of DOCSIS specifications define all levels of communication including the physical layer, media access control layer, and an application interface layer.
Many CMs share the bandwidth of the same coaxial cable. The coaxial cable has a bandwidth of approximately 1 GHz, which is divided into multiple channels. The spectrum consumed by a given channel is defined by its center frequency and width. Each defined channel is typically shared by many CMs. In the downstream direction, from the CMTS to the CM, the CMTS uses time division multiplexing to send data to all CMs using a unique address to send data to a particular CM. In the upstream direction, from the CM to the CMTS, many CMs must share the same channel. To accomplish this, the CMTS schedules time slots for each CM in a control structure known as a MAP. A given CM is only allowed to send data during its time slot. Synchronization signals from the CMTS to the CM keep the different CMs synchronized. Within a channel, Quadrature Amplitude Modulation (QAM) is used to represent the data on the coaxial cable.
The HFC plant, especially the coaxial portion, is subject to many different types of impairments that degrade the quality of the signal. These impairments are typically caused by problems such as loose or corroded connections, unterminated lines, faulty equipment, and other noise caused by sources such as motors and lightning. Some types of noise such as Additive White Gaussian Noise (AWGN) are present all of the time. Another type of noise, known as burst or impulse noise, persists for a relatively short period of time. These noise sources can cause the decoded constellation point to move from its ideal position, thereby corrupting the transmission of data. The direction of movement and the amount of movement depend upon the phase of the noise source relative to the signal and the magnitude of the noise.
DOCSIS defines multiple mechanisms for dealing with different kinds of noise. For example, Reed Solomon (R-S) Forward Error Correction (FEC) is a redundancy code used for recovering multiple bytes of corrupted data. R-S FEC is useful for combating both AWGN noise and burst noise. Interleaving is another mechanism that interleaves different R-S FEC codewords such that a burst of noise impacts a small portion of many codewords instead of a larger portion of one codeword. This technique is also useful for combating burst noise. Both R-S FEC and interleaving have various defined parameters that can be adjusted for custom tuning to the conditions of the particular HFC plant. Therefore, measuring the characteristics of the noise in the HFC plant is an important tool for determining the proper corrective action for operators of such systems.